Inks are often of one of two types, depending on the type of colorant used. Dye-based inks often comprise a dye dissolved in a liquid medium. Pigment inks comprise a pigment dispersed in a particulate form in a liquid medium. Pigment inks tend to have better ozone-fastness and light-fastness than dye-based inks. However, because the pigment is in the form of fine particles there is a tendency for the particles to agglomerate or flocculate whilst the ink is being stored and/or whilst the ink is being used (e.g. printed). Such agglomeration or flocculation before the ink has been printed onto a substrate is highly undesirable, particularly in ink jet printing inks where the printer nozzles are very small and are susceptible to blockage by any oversized particulate matter. Thus, in the ink jet field a great deal of effort has been spent attempting to provide sub micron sized pigment dispersions and increase the colloidal stability of these pigment dispersions.
It is also desirable to provide pigment inks which offer high optical density (OD), especially when printed onto plain paper.
Pigment dispersions are often colloidally stabilised by means of a dispersant.
In our own studies on dispersant stabilised pigment inks we have found that it is particularly difficult to prepare inks which simultaneously exhibit good colloidal stability and high OD on plain paper. For example, we have found that dispersant stabilised pigment inks known in the art having a high colloidal stability provide a low OD when printed on to plain paper and vice versa.
We have also found that the few dispersant stabilised pigment inks which do provide inks exhibiting high OD when printed onto plain paper tend to also use dispersants which require significant and undesirably high amounts of organic solvent to assist in dissolving/dispersing the dispersant, for example in the pigment dispersion or comminution step.
Further it is desirable that a dispersant is effective in the comminution process. Quicker comminution (e.g. milling) to submicron particles sizes saves substantial energy and it may also result in less pigment particles having a particle size markedly smaller than the target size. Particles much below the target size are often referred to as fines.
There is also a need to prepare pigment dispersions which can tolerate liquid vehicles containing significant amounts of organic liquids without flocculating the particulate solid.
Commercially, there still remains a need for dispersants and dispersions which can be used to prepare pigment inks and which solve, at least in part, one or more of the abovementioned problems.